Physical-Vapor-Deposited Metal Oxide Thin Films for pH Sensing Applications: Last Decade of Research Progress

Abstract

In the last several decades, metal oxide thin films have attracted significant attention for the development of various existing and emerging technological applications, including pH sensors. The mandate for consistent and precise pH sensing techniques has been increasing across various fields, including environmental monitoring, biotechnology, food and agricultural industries, and medical diagnostics. Metal oxide thin films grown using physical vapor deposition (PVD) with precise control over film thickness, composition, and morphology are beneficial for pH sensing applications such as enhancing pH sensitivity and stability, quicker response, repeatability, and compatibility with miniaturization. Various PVD techniques, including sputtering, evaporation, and ion beam deposition, used to fabricate thin films for tailoring materials' properties for the advanced design and development of high-performing pH sensors, have been explored worldwide by many research groups. In addition, various thin film materials have also been investigated, including metal oxides, nitrides, and nanostructured films, to make very robust pH sensing electrodes with higher pH sensing performance. The development of novel materials and structures has enabled higher sensitivity, improved selectivity, and enhanced durability in harsh pH environments. The last decade has witnessed significant advancements in PVD thin films for pH sensing applications. The combination of precise film deposition techniques, novel materials, and surface functionalization strategies has led to improved pH sensing performance, making PVD thin films a promising choice for future pH sensing technologies.

Keywords: metal oxide; pH sensor; sensitivity; thin film.

Figure 1

Application areas of pH sensors.

Figure 2

SWOT analysis of sputtering, evaporation, and ion beam deposition techniques that are commonly used for thin film preparation. Note: This SWOT chart is a synopsis of thin film deposition techniques. However, specific advantages and limitations may vary depending on the deposition system, process parameters, and materials used.

Figure 3

The group of materials that are frequently used to design and synthesize pH sensors/sensing devices using PVD techniques is presented in the periodic table schematically. Note that most of the metal oxides used in the last decade for pH sensor development belong to the transition metal group, as indicated by the yellow and green circles in this periodic table presentation.

Figure 4

A presentation of fabrication to the characterization of RuO₂-based pH sensing electrodes: a simple schematic diagram of a sputtering process where the material deposition process optimization was required to obtain the best material properties (a); the structural characterization (X-ray diffraction and scanning electron microscope image) is an essential part of developing and confirming the high-quality material layer grown (b,c); the developed material testing setup to obtain results of pH measurement (d); the pH stability testing result for a RuO₂ layer (e); and development and testing results of RuO₂ pH sensing electrodes on flexible substrates (f) [22,23,27].

Figure 5

A glimpse of ZnO-based pH sensing electrode development: effects of pH on the optical properties of ZnO film (a), thus leading to determine the pH dependence of the bandgap of ZnO film (b); photolithography-assisted developed ZnO-coated ultra-sensitive silver integrated electrode (IDE), and the steady-state current response performance with different pH solutions (c,d); development of and performance comparison in terms of average sensitivity for ZnO and aluminum doped ZnO (AZO)-based pH electrodes (e), and the sensitivity improvement on Magnesium (Mg)-doped ZnO pH sensing electrodes (f) [47,50,51].

Figure 6

TiO₂-based pH sensing electrodes development: microstructure evaluation of TiO₂ films deposited at different temperatures (a), working principle of TiO₂-base pH electrode (b), and the observed effects of deposition process parameters (such as film growth temperature) on the hysteresis characteristics of pH loop (c) [54,56].